This invention relates to packet data communication networks, and more particularly to a way of handling packets through a hub, switch or router for a network, using a special header attached to the packets.
Packet data communication networks of type using Ethernet, token ring, or FDDI technologies, or other network varieties, hubs are used for switching or routing, or for bridges to additional segments of the network. A bridge or router in a computer interconnect system is shown in U.S. Pat. No. 5,020,020, assigned to Digital Equipment Corporation. It is typical to use address translation in a bridge or router of this type, as shown, for example, in U.S. Pat. No. 4,933,937. The need to translate addresses is due to the address length. Some protocols or system specify a 48-bit source and destination address so that a globally unique address is provided. However, for efficient use of resources at a local segment of a large network (as within a bridge), it is advantageous to use smaller address fields instead of 48-bit addresses, for efficiency in bit-count of messages as well as efficiency in processing and storage. For this reason, while the 48-bit addresses are carried in the packet throughout its lifetime, shorter addresses are generated for local routing and processing. Thus, a translation mechanism is provided to allow switching between global and local addresses.
Various other information may be locally useful in a bridge or router that need not be carried by a packet throughout its lifetime. For example, the network segments (links) may be locally named (assigned an identifying number) so that packets may be routed by link number instead of or in addition to their local destination node address. The packets may be assigned a priority or "service class" for local use, aside from the priority attached to the packet by the protocol being implemented. Status information may be locally useful, but not part of the packet protocol and not necessary for transmission with the packet throughout the network. For example, local congestion information, or results of address translations, or end-of-message information, are types of such status information.